This narrowing typeguard effect bleeds into subsequent statments on a type with bivariant type-parameter

TypeScript Version: 3.4.0-dev.201xxxxx (although 3.3 is also impacted)
Search Terms: this type guards

If we create a type-guard that narrows this on a type that has a type-parameter that is present only in a bivariant position, the effect of the type guard persists outside of the guarded block.

Code

type GetKnownKeys<G> = G extends GuardedMap<infer KnownKeys>  ? KnownKeys: never;
interface GuardedMap<KnownKeys extends string> {
    get(k: KnownKeys): number;
    has<S extends string>(k: S): this is GuardedMap<S | GetKnownKeys<this>>;
}

declare let map: GuardedMap<never>;

map.get('bar') // err, as expected
if (map.has('foo')) {
    map.get('foo').toExponential(); // ok as expected
    if(map.has('bar')) 
    {
        map.get('foo').toExponential(); // ok as expected
        map.get('bar').toExponential(); // ok as expected
    }
        
    map.get('bar').toExponential(); /// OK!?!?! WHY ?!
    
}
map.get('bar') // OK ?!

Expected behavior:
Type guard only impacts the guarded block.
Actual behavior:
The effect of the type guard bleads into all subsequent statements. (marked with OK!?!?! and OK?!)

Note: With strictFunctionTypes on, declaring get as get: (k: KnownKeys) => number; makes the code work as expected.

Playground Link: link

Related Issues: Similar to #14817

Found this while playing with a solution for #9619

提供されたGuardedMapの例を使用して、リンク先のTypeScript Playgroundでnarrowing leakを再現します。thisのtype guardと反変な型パラメーターに対するコンパイラーのcontrol-flow処理を追跡します。narrowingがガードされたブロック内にとどまり、ブロック後のmap.get('bar')が拒否され、回帰テストによるカバレッジが追加されれば完了です。